feat(virtual-core): add deferLaneAssignment option

[2wheeh]

🎯 Changes

resolves #1114

Adds deferLaneAssignment option to defer lane caching until items are measured via measureElement.

#1080 introduced lane assignment caching based on estimateSize for visual stability. However, this broke a core use case for masonry layouts where lane assignments should be based on actual measured sizes, not estimates.

This PR adds a new option deferLaneAssignment (default: false):

• When false (default): Current behavior - lanes cached immediately from estimateSize
• When true: Lanes are calculated but not cached until first measurement, then cached based on actual sizes

✅ Checklist

• I have followed the steps in the Contributing guide.
• I have tested this code locally with pnpm run test:pr.

🚀 Release Impact

• This change affects published code, and I have generated a changeset.
• This change is docs/CI/dev-only (no release).

[changeset-bot]

🦋 Changeset detected

Latest commit: 7a2e014

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 7 packages

Name	Type
@tanstack/virtual-core	Patch
@tanstack/angular-virtual	Patch
@tanstack/lit-virtual	Patch
@tanstack/react-virtual	Patch
@tanstack/solid-virtual	Patch
@tanstack/svelte-virtual	Patch
@tanstack/vue-virtual	Patch

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